By directly measuring the forces between large molecules or between membranes we are learning finally how these interactions allow the formation of molecular assemblies. Beginning with the discovery that hydration or solvation forces are the most important interactions encountered during the last few nanometers approaching contact, we are now systematically investigating the molecular features that govern surface hydration. Hydration forces are very sensitive to relatively small changes in composition of lipid polar groups or of small molecules bound to the macromolecular surface. Following our measurement of the change in molecular disorder accompanying changes in molecular interaction with separation, we have derived a model of molecular motion in the force field of neighboring molecules. There has finally been a connection established between molecular force and motion. On the theoretical side also, we have analyzed the fluctuations of ionic charge and found that the resulting charge fluctuation forces, analogous to van der Waals forces, create strong attractive forces that explain the puzzling "long range hydrophobic interaction" between non-polar surfaces. Earlier ideas about this interaction had to assume improbable perturbation of water solvent hundreds of Angstroms from the surface.